The present invention relates generally to improved method and apparatus for filtering solids from liquid suspensions, and more particularly relates to a vacuum filtration system which achieves improved filtering of pulp fibers from pulp slurry during the continuous formation of pulp sheets.
There are a variety of industrial and manufacturing processes which require filtering of dispersed solids from liquid suspensions. Such filtering functions are of particular importance in the manufacture of paper, paperboard, and felt.
In the manufacture of paper and paperboard products, wood is broken into chips which are then digested and bleached to provide a pulp slurry. The pulp fibers in this slurry are dispersed, typically in water, and often in the presence of additives in order to improve the paper manufacturing process or the characteristics of the paper product. Examples of such additives include drying agents, wet and/or dry strength enhancers, antistatic agents, and the like. Typically, the pulp slurry is highly diluted, for example, about 4% solids, and thus is often called "dilute stock." The pulp solids are removed from the pulp slurry by filtering the liquid portion of the slurry (i.e. the filtrate) through a screen or similar filter device to produce a wet sheet of pulp that is deposited on the surface of the filter. While deposited on the filter surface, the wet pulp sheet can be washed using water in order to remove the various additives. The pulp sheet is then removed from the filter surface for further drying and subsequent processing into a final paper product. Similar processing techniques are used in the manufacture of felt and like articles.
One method for filtering pulp fibers from pulp slurry involves the use of a cylinder drum disposed horizontally and partially submersed in a tank containing a desired amount of pulp slurry. The cylindrical wall of the drum supports a filtering surface, often made from a metallic screen material, on which the pulp collects and forms a wet pulp sheet as the drum is rotated. The pulp sheet is removed continuously from the rotating filter surface by a take-off device and, in some applications, the wet pulp sheet may be squeezed between the filter surface and a roller to remove additional filtrate prior to removing the pulp sheet from the cylinder drum. Within the interior of the cylinder drum, a series of filtrate passages are provided for the purpose of collecting filtrate from filtrate inlet ports which extend longitudinally along the filter surface. Filtrate passing through each filtrate inlet port is directed along a filtrate passage to either the center or the end of the drum, from where it is removed. To minimize environmental impact, the collected filtrate is typically recycled and used in the preparation of fresh pulp slurry and/or clean water for pulp washing operations. Hitherto, two principally different methods have been used to create a pressure differential at the filter surface in order to conduct filtrate across the filter surface and thereby deposit pulp fibers on the filter surface.
The first method, which was more popular in earlier times than at present, uses only the force of gravity to induce the flow of filtrate across the filter surface and deposit pulp fibers thereon. This method, known as "beater washing", involves the use of a "beater washer". In general, the "beater washer" cylinder is lowered into the flow of dilute stock and is then rotated by external gearing. Typically, the outside surface of the cylinder is covered with above 60 or 80 mesh (per inch) wire. As the cylinder rotates, the hydraulic pressure differential (i.e. head) created across the outside surface of the cylinder causes the dilute stock to flow into the cylinder, depositing pulp fibers on the outside of the screen. The major drawbacks to this method are that pulp washing is quite slow and uses water inefficiently.
The second and more popular method of creating a pressure differential across the filter surface involves the use of a "vacuum washer". In general, the vacuum washer includes a cylinder which is rotated about its axle within a tank of pulp slurry. Typically, the rotary cylinder contains a series of filtrate conduits or compartments within the interior of the cylinder. Each of these filtrate conduits has a filtrate inlet port which extends along the longitudinal length of the cylinder, and a filtrate outlet port which is connected to a filtrate outlet tube. This tube rotates about a stationary vacuum tube that is connected to a vacuum or suction pump. In general, the portion of each filtrate conduit extending from the filtrate inlet port to its associated filtrate outlet port is rectilinear and the filtrate inlet and outlet ports are radially aligned. Disposed over the filtrate inlet ports is a filter surface, typically constructed from wire and mesh screening. In order that only a selected number of filtrate conduits, are connected to the vacuum source at a time, the vacuum tube has a control valve. As the cylinder rotates with the vacuum pump activated, there is always a first stationary region over the filter surface where vacuum pressure is present and a second stationary region over the filter surface where vacuum pressure is absent. The second stationary region is located where the pulp sheet is to be removed, whereas the first stationary region is located where filtrate flow is to be induced for purposes of achieving pulp fiber deposition dewatering and filtrate removal operations.
While prior art vacuum washers have performed a number of important functions, they have, however, suffered from a several significant shortcomings and drawbacks.
Specifically, the pressure differential required to induce filtrate flow across the filter surface of prior art vacuum washers has been provided primarily by vacuum forces created by an associated vacuum pump. In addition, the column of filtrate contained within each filtrate conduit disposed below the pulp slurry level in the vessel, presents a negative hydrostatic pressure or head which the vacuum forces of the vacuum pump must overcome in order to suction filtrate through the filter surface. Consequently, filtrate conduits which contain columns of filtrate that are highly aligned with the gravitational force field, are subjected to the greatest amount of negative hydrostatic pressure, substantially decreasing the effective vacuum forces of the vacuum pump along these filtrate conduits and during vacuum start-up operations.
Moreover, as the wet pulp sheet on the filter surface is sprayed with water during washing operations, the pressure drop presented by the pulp sheet itself reduces the efficacy of vacuum pressure supplied to the filter surface during subsequent dewatering operations.
The foregoing illustrates limitations known to exist in present vacuum filtration systems and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.